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Some NHS hospitals treating such patients refused to provide any data, while among the 35 hospitals, some would only disclose the figures for those children admitted to wards after becoming dangerously emaciated - excluding those undergoing psychiatric therapy as outpatients.

Susan Ringwood, chief executive of eating disorders charity B-eat said the figures reflected alarming trends in society, with young children "internalising" messages from celebrity magazines, which idealised the thinnest figures.

"A number of factors combine to trigger eating disorders; biology and genetics play a large part in their development, but so do cultural pressures, and body image seems to be influencing younger children much more over the past decade," she added.

Research carried out by the charity with the Brownies found that even by the age of seven, girls who looked at outline drawing of women thought the thinner ones were happier and more popular than those with slightly larger outlines.

Mrs Ringwood said young girls felt increasingly frightened by the prospect of gaining weight in puberty. She said: "Children are receiving very pernicious messages.

"The ideal figure promoted for women these days is that of a girl, not an adult women. Girls see the pictures in magazines of extremely thin women and think that is how they should be.

"That can leave them fearful of puberty, and almost trying to stave it off."

In 2009, Kate Moss, the supermodel, was accused of encouraging girls to become anorexic when she said she lived by the phrase 'nothing tastes as good as skinny feels' - a mantra of pro-anorexic groups.

While disorders among men are increasing, cases involving boys were often sparked by specific incidents, such as being bullied because of their weight, she said.

Separate research published in the British Journal of Psychiatry earlier this year suggests one in five children diagnosed with an eating disorder have a history of early feeding problems, such as fussy eating.

Almost half of those diagnosed with disorders by the age of 12 had a close family member with a mental health problem such as anxiety or depression.

The study by the University College London's Institute of Child Health found more than 80 per cent of cases involved girls, with anorexia – which involves drastically reducing the intake of food and drink – far more common than bulimia – which involves sufferers binge eating and then making themselves sick.

Last year a survey of women suffering from anorexia found almost half said they had a problem with food by the age of 10.

Experts say there is no clear relationship between "fussy eating" and the later development of a disorder.

But Mrs Ringwood said some particular behaviours with food – such as cutting it into tiny pieces, or insisting that foods were eaten separately, could indicate early signs of a more significant problem.

Other types of behaviour which had nothing to do with food could provide some clues, she said.

"If children become very rigid in their routines and get upset if changes are made that can be an indicator of the type of anxiety associated with disorders," she said.

Even young children who were trying to restrict their diet could be stealthy about it, she said, hiding food up their sleeves at mealtimes, in order to throw it away.

Georgie Bevan, from Bury St Edmunds in Suffolk, was 11 when signs first appeared of a problem with food.

For the boarding school pupil, it began with a class lecture on healthy eating, which appeared to trigger an obsession with cutting out fat from her diet.

When she returned home for the summer holidays, parents Charlotte and Christopher, who run a farm, were shocked to see how thin she had become.

Shortly after she was diagnosed, aged 12, she reached a point of refusing all food and drink and had to be admitted to a psychiatric hospital for several weeks.

At first, she seemed to make a quick recovery.

Mrs Bevan, 46, said: "She went back to school for the autumn term. Ten days later, the school phoned and said she had stopped eating. It was heartbreaking. We didn't know what to do."

Two years on, after undergoing family therapy, and being sent to a day school, Georgie is a healthy weight, and her parents believe she has made a full recovery.

Her mother said: "When something like this happens of course you ask yourself questions – is it something we did? I'm not a perfect mother but I think I did a good job.

"From everything we know, the predisposition to eating disorders is genetic but it is the environment that pulls the trigger."